1. Field of the Invention
This invention relates to an optical pickup apparatus, and more particularly to an optical pickup apparatus which produces a laser beam for reading out information recorded on an information recording medium.
2. Description of the Prior Art
In recent years, optical disks having a high recording density and capable of recording large volumes of information have been finding many uses in various fields of application. Because of the high recording density of an optical disk, an optical pickup device for reading out information from the optical disk is provided with an optical system for focusing the laser beam into a very small spot.
FIG. 7 is a diagram illustrating the construction of a conventional optical pickup apparatus. The optical pickup apparatus of FIG. 7 comprises a laser device 21, a diffraction grating 22, a beam splitter or half mirror 23, a collimating lens 24, an objective lens 25, a plano-concave lens 27, and a photodetector 28, and is used for reading information recorded on an optical disk 26 in which information is stored in the form of pit tracks T (FIG. 8).
In this optical pickup apparatus, a laser beam emitted from the laser device 21 is separated by the diffraction grating 22 into three beams, a zero-order diffracted beam and first-order diffracted beams in the positive and negative directions. Hereinafter, these two first-order diffracted beams are referred to as "plus and minus first-order diffracted beams". The zero-order and first-order diffracted beams are then reflected by the beam splitter 23, converted through the collimating lens 24 into parallel beams, and focused through the objective lens 25 to be projected onto the optical disk 26. The beams reflected from the optical disk 26 pass through the objective lens 25, the collimating lens 24, the beam splitter 23 and the plano-concave lens 27, and impinges onto the photodetector 28 which converts incident light beams into electric signals.
Formed on one surface 22a of the diffraction grating 22 are parallel grooves having a pitch of approximately 30 .mu.m and a depth of approximately 0.3 .mu.m. Two subbeams (plus and minus diffracted beams) are formed by the parallel grooves. By detecting the difference in the reflection amount between the two subbeams, it is possible to detect whether the zero-order diffracted beam is correctly tracking the center of a recording track on the optical disk 26. The detected signal is also used as a tracking servo signal.
In an optical pickup apparatus described above, since the recording tracks are each formed in a very narrow width of 1 to 2 .mu.m, a laser spot S projected onto the optical disk must be focused into a diameter of approximately 1 .mu.m to match the recording track width. For that purpose, the objective 25 is required to have a high numerical aperture (NA) so that the size of the laser spot S is reduced while increasing the intensity of the beam.
However, if the laser spot S is reduced to an extremely small size, an Airy ring (secondary maximum) S.sub.1 is produced around the converged spot S, as shown in FIG. 8. On the other hand, if the Airy ring S.sub.1 extends to hit the adjacent recording tracks, the problem of crosstalk is caused.
To overcome the above-mentioned problem, there has been proposed a configuration in which the diffraction grating 22 is provided with a light attenuation filter or neutral density filter (ND filter) 29 which partially covers the surface 22b opposite to the grating surface 22a (for example, Japanese Laid-open Patent Publication No. 2,270,034). In the prior art optical pickup apparatus, as shown in FIG. 9A, the center portion 22bc not covered by the ND filter 29 has a transmittance which is different from that of side portions 22bs covered by the ND filter 29. More specifically, as shown in FIG. 9B, the center portion 22bc is provided with a transmittance of 100%, while a transmittance of 10 to 40% is set for the side portions 22bs. In FIG. 9A, the laser light spot is indicated by a circle L. By setting the transmittances in this manner, the distribution of the laser beam intensity entering the objective lens 25 can be adjusted so that a laser beam of high intensity is obtained at the center of the spot S while reducing the laser beam intensity at the periphery thereof. This results in a weakened intensity of the Airy ring of the laser spot S, thereby hindering the occurrence of crosstalk even in the case of a narrow recording track width.
In the prior art optical pickup apparatus, however, the amount of the laser beam is partially reduced by the absorption or reflection of light caused by the ND filter 29, resulting in that a partial amount of the laser beam is not used. That is, the utilization efficiency of light in the prior art apparatus is inferior.